This section introduces aspects that may facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In multiple-input multiple-output (MIMO) wireless communication systems, both of the transmitter and the receiver use antenna arrays to provide a rich diversity and a large communication capacity. The downlink transmission process in Long Term Evolution (LTE) and LTE-Advanced (LTE-A) may comprise steps of: generating code words by channel coding and modulating of data from an upper layer; layer mapping of different code words; and precoding the layer mapped data so as to transmit from the antenna array. The number of layers is the degree of freedom of the MIMO channel matrix, which is also referred to the rank of the MIMO system.
A LTE enhanced Node B (eNB) performs downlink and uplink link adaptation so that the data transmission matches with the radio link quality experienced by a user equipment (UE). To be more specific, the eNB dynamically determines the number of layers, or rank, in case MIMO is enabled, as well as selects a Modulation and Coding Scheme (MCS), which includes Transport Block Size (TBS) and modulation scheme.
To facilitate downlink link adaptation in LTE/LTE-A, the UE reports its measured Channel State Information (CSI) to the eNB, such as Channel Quality Indicator (CQI) and Rank Indicator (RI) when MIMO or spatial multiplexing is enabled.
However, CSI is often with errors, i.e., either over-estimated or under-estimated. To cope with errors in the reported CQI, the eNB often adopts outer-loop link adaptation, i.e., adjusting CQI based on received downlink Hybrid Automatic Repeat Request Acknowledgement (HARQ-ACK) feedback from the UE. After CQI adjustment, the selected MCS better reflects the UE experienced radio link quality on Physical Downlink Shared Channel (PDSCH) reception.
In contrast to CQI adjustment, the eNB usually follows RI reported from the UE and determines the rank accordingly in the MIMO transmission. However, like any CSI, RI reported from the UE is often with errors, i.e., over-estimated or under-estimated.
In case of un-shifted Cell specific Reference Symbol (CRS) configuration in a LTE network deployment, the reception of CRS at UE side is interfered by all neighbouring cells, while PDSCH reception does not always experience the same interference since the interference level depends on the load situation in neighbouring cells. For example, in an un-loaded network with un-shifted CRS configuration, based on measured CRS quality UE under-estimates the radio link quality on PDSCH reception and reports more RI=1 than it should do. Consequently, the eNB only schedules rank 1 transmission even though the radio condition allows rank 2 transmission for PDSCH, i.e., radio spectrum is under-utilized and user throughput is lower than what it can be achieved. The miss-match between CRS quality and PDSCH quality could also happen in TM8 (Transmission Mode 8, dual-layer beam-forming in TD-LTE) transmission, where CSI report based on CRS does not grasp the beam forming effect on PDSCH. Another factor causing the improper rank is the estimation error of the rank in the UE or the impairments in UE implementation.
On the other hand, field tests revealed that sometimes TM3 (Transmission Mode 3, open-loop MIMO) transmission with rank adaptation is worse than single-stream transmission at low to medium signal to interference and noise ratio (SINR). It is suspected that worse MIMO performance is probably caused by wrongly reported RI, i.e., RI=1 should have been reported instead of RI=2.
In “Downlink Transmission Mode Selection And Switching Algorithm For LTE”, Shubhodeep Adhikari, LTE Systems Engineering Group, Motorola Networks, Bangalore, India, Third International Conference on Communication Systems and Networks (COMSNETS), 2011, it is disclosed that making MIMO transmission mode selection at the eNB based solely on mobile feedback can result in significant throughput loss, and an algorithm is described which can prevent such throughput reduction. The algorithm works by enabling the eNB to selectively override the mobile's channel feedback and make a better choice of transmission mode.
In PCT patent application WO2012044236, a mechanism that may be used as a tool for the sending node or eNB to perform rank override of the UE or receiving node feedback reporting is provided. Specifically, by setting a very high Power Measurement Offsets (PMO) value or scaling factor for a certain rank, the receiving node or UE can be made to always report only that rank.